1. Field of the Invention
The present invention generally relates to the field of phase shifting interferometry. In particular, the invention relates to a phase shifting interferometric method, a phase shifting interferometer apparatus and applications using such method and apparatus. One particular application of the phase shifting interferometry is in the field of testing and manufacturing an object such as an optical element.
2. Brief Description of Related Art
An optical element may comprise, for example, an optical component such as an optical lens or an optical mirror used in optical systems, such as telescopes used in astronomy, and systems used for imaging structures, such as structures formed on a mask or reticle, onto a radiation sensitive substrate, such as a resist, in a lithographic method. The success of such an optical system is substantially determined by the accuracy with which the optical surface can be machined or manufactured to have a target shape determined by a designer of the optical system. In such manufacture it is necessary to compare the shape of the machined optical surface with its target shape, and to determine differences between the machined and target surfaces. The optical surface may then be further machined at those portions where differences between the machined and target surfaces exceed e.g. predefined thresholds.
Interferometric apparatuses are commonly used for measurements of optical surfaces. Examples of such apparatus are disclosed in U.S. Pat. Nos. 4,732,483, 4,340,306, 5,473,434, 5,777,741, 5,488,477. The entire contents of these documents are incorporated herein by reference.
Conventional phase shifting interferometry “PSI” is a data collection and analysis method for obtaining a high accuracy result from interferometric measurements. Various methods and background information of phase shifting interferometry is available from the articles by J. E. Greivenkamp et al. at “Phase shifting interferometry” in Optical Shop Testing edited by D. Malacara, 2nd edition, Wiley Interscience Publication (1992), and Catherine Creath “Temporal Phase Measurement Methods” in Interferogram Analysis, Digital Fringe Pattern Measurement Techniques, edited by D. W. Robinson et al., Institutes of Physics Publishing, Bristol and Philadelphia, 1993. These articles are incorporated herein by reference.
In conventional phase shifting interferometry, the relative phase between interfering beams of the interferometer is changed in a predetermined manner, and a plurality of interferograms is recorded while changing the relative phase. The knowledge on the phase settings is used for the analysis of the interferograms to determine the desired distribution of relative phases across the interfering beams.
It has been found that in some situations the resulting accuracy of the interferometric measurements using PSI is not as high as expected.